1. Field of the Invention
The present invention relates to an ink, and particularly to an ink suitable for use in recording on so-called plain paper such as woodfree paper, an ink-jet recording method using such an ink, and ink-jet recording instruments to which the ink is applied.
2. Related Background Art
It has heretofore been discovered that since polyamines such as polyethyleneimine act as cationic polyelectrolytes in an aqueous solution containing a dye and hence have affinity (weak bonding) for azo dye molecules having an anionic group such as a sulfonic group, and also have strong affinity (van der Waals' forces and hydrogen bond to cellulose, etc.) for recording media, they can improve the water fastness of inks used in ink-jet printers.
For example, an ink disclosed in France Patent No. 1480068 contains a polyethyleneimine having a molecular weight of 500 to 100,000. This polyethyleneimine contains a primary amine on its molecule. Most of direct dyes and acid dyes used in inks for ink-jet printers contain at least one azo bond. Now, such an ink has involved a problem that this azo bond is reduced by the primary amine of the polyethyleneimine molecule in the aqueous solution during the storage of the ink, and so the dye is decomposed to make the ink useless. On the other hand, the ink according to the present invention, which will be described subsequently, is similar to such an ink, but can retain its initial performance for a long period of time because the dye used therein is different from that of the France patent, and is not decomposed even after its storage.
On the other hand, U.S. Pat. No. 4,197,135 discloses an ink composition having good water fastness, which comprises at least one water-soluble dye and a polyamine containing at least 7 nitrogen atoms in its molecule. According to this U.S. patent, both good water fastness and good ink storability can be achieved at the same time. However, the polyamine used is limited to polyamines containing no primary amine. Polyamines containing a secondary amine or a tertiary amine also have a sure effect on water fastness. However, their performance is somewhat poor compared with the polyamines containing a primary amine. Namely, the ink composition according to U.S. Pat. No. 4,197,135 has not been said to completely satisfy the water fastness.
Further, U.S. Pat. No. 4,659,382 discloses an ink composition comprising a major amount of water, a hydroxyethylated polyethyleneimine polymer and a dye compound and having good water fastness, said polymer containing about 65 to 80% of a hydroxyethyl group. In this case, the polymer is hydroxyethylated, so that its hydrophilicity becomes higher, and the storability of the ink is improved. However, the ink has involved a problem that the water fastness is somewhat deteriorated correspondingly.
Furthermore, U.S. Pat. No. 5,100,470 discloses an ink composition comprising a major amount of water, a water-soluble organic solvent, a polyalkylenepolyamine containing nitrogen atoms up to five and a dye. However, since the improvement of water fastness by the polyalkylenepolyamine directly correlates to the number of nitrogen atoms. The effect of the polyalkylenepolyamine on the water fastness is scarcely exhibited unless it contains at least 7 nitrogen atoms. Therefore, the polyamine according to this U.S. patent has some effect to improve water fastness on only limited dyes.